This invention relates to a method for analyzing metal strain caused by deformation of sheet metal, which analysis comprises measuring distortion in a grid applied to the sheet surface prior to deformation. More particularly, this invention relates to decorating a metal sheet surface with a grid featuring uniform solid circles that are stretched into ellipses during sheet deformation and thereafter readily measured to calculate strain in the underlying metal.
Sheet metal components are fabricated by stamping a flat sheet between suitable dies. Improved die design is possible by understanding the metal strain produced by stamping at various locations of the sheet. A preferred method for analyzing the strain involves printing on the flat sheet a grid pattern of uniform small circles of predetermined diameter. During metal forming, the circles are stretched into ellipses. By determining the change in dimensions for each ellipse, the strain of the underlying metal may be calculated. Accurate human measurement of each ellipse is tedious and timeconsuming, typically requiring up to two minutes per point. However, coassigned U.S. patent application Ser. No. 098,260, filed Nov. 28, 1979, and entitled "Method and Apparatus for Automatically Determining Sheet Metal Strength", teaches a method for determining the ellipse dimensions and calculating the strain that is adapted to be carried out by a computer in a fraction of the time. The method comprises photographing the elliptical pattern on the sheet metal surface with a solid-state instrumentation camera. The computer divides the picture into light and dark regions, determines the boundary points between the regions and fits the boundary points to an ellipse. The strain is then computed from the major and minor axes of the ellipse.
One method of printing the circle grid comprises electro-etching. The desired pattern is defined with an electrolyte-soaked silkscreen and is etched in the metal by suitably passing a current through the screen between the sheet and a roller counterelectrode. Grids produced by electro-etching have intolerable variations in the circle diameters and poor circle (and thus ellipse) edge definition. This reduces the accuracy of the strain measurements. In addition, etching sufficient to produce a visible pattern may affect the sheet metal formability. Another grid-printing method comprises coating the sheet with a polymeric photoresist and exposing to ultraviolet light through a suitable mask to define the circles. The usually clear polymer must be dyed to make the pattern visible, particularly for photographing. Although photoprinting improves circle uniformity, the circles are frequently abraded or wrinkled during stamping because the polymer-metal adhesion is not sufficient to withstand the wiping die action. In addition, the polymer tends to deteriorate, particularly where high temperatures are encountered during stamping.
Therefore, it is an object of this invention to provide a method for decorating a surface of a metal sheet with a sharply defined, dimensionally accurate grid of a pattern suitable for strain analysis in the deformed metal, preferably an array composed of uniformly spaced solid circles of predetermined diameter. The grid does not significantly affect the metal sheet formability, and tightly adheres to the surface without deteriorating during deformation to thereby form a distorted pattern on the formed sheet, preferably of ellipses. After deformation, the distorted grid develops a high-contrast pattern having accurately locatable boundaries that is readily photographable and adapted for computer strain analysis.